Conventional ad-hoc solutions do not meet the need for strong authentication as they merely check for internal consistency of signals, such as eliminating GNSS positions and velocities that are inconsistent with sensor readings or eliminating signal levels which may be too high caused by attempts to overwhelm a receiver with much stronger fake signals than the real signal.
Another conventional approach is to extract the Y code from broadcast satellites by pointing high gain antennas at each satellite, to compensate for the lack of correlation gain, which may be obtained by correlating with a known version of the local code, and by estimating the Y code per satellite in real time. This approach collects raw data from the satellite, and correlates them with another sample coming from the receiver whose position needs to be authenticated. Knowing the location of each satellite from ephemeris by blindly tracking the satellite location with steerable high gain antennas, inphase and quadrature (I/Q) samples are collected. An antenna gain of around 30 dB may be generally sufficient for this approach (which may be obtained, for example, by a parabolic antenna about 2 to 3 meters in diameter). The satellite selection is done by spatial diversity, for example assuming that the antenna is correctly pointed to the satellite and that no other satellite is crossing the high gain antenna beam. However, this approach can create ambiguity with regards to which signal is being correlated with and may require specific antenna configurations.
Another problem with the above conventional approach is that coarse acquisition (C/A) code is also received along with Y code as it is orthogonal to the Y code. The I/Q samples need to be rotated orthogonally with the C/A code to have only the Y code subsisting into the raw signal. As a result, with the conventional approach, each satellite is tracked separately by an individual steerable high gain antenna, which significantly increases its cost.
In addition, when the satellites cross paths, there may be ambiguity about which satellites are being tracked, and the authentication with these satellites may need to be temporarily stopped. In some conventional solutions, in order to reduce cost, only one Y code signal can be authenticated, and the rest is considered genuine. These solutions limit the level of protection that can be offered as they do not cover the case of real collected signal (comprising all Y codes), with fake C/A codes superimposed.
Therefore, there is a need for methods and systems that can address the above issues of conventional solutions.